Recently, inkjet printers have been rapidly developing their market owing to their features such as printability on regular paper, easy colorization, compact size, inexpensive price, and low running cost.
Typical properties required for inkjet recording ink are as follows: color tone, image density and bleeding to achieve high image quality; dissolution or dispersion stability, preservation stability, and discharge stability of colorant in the ink to achieve reliability; water resistance and light resistance to secure the storage life of recorded images; rapid drying of ink to achieve high-speed printing. Various improvements have been proposed heretofore in order to satisfy these requirements.
Initially, colorants of the inkjet recording ink were mainly based on dyes owing to their bright coloration and high reliability; recently, ink components containing pigments such as carbon black have gained attention for the purpose of making the recorded images light- and water-resistant.
Further, the recent trend is to make the size of ink drops smaller in order to achieve high-quality images and high-speed printing; therefore, the nozzle diameter also tends to be reduced.
However, it is considerably difficult to assure discharging stability of printers when pigments are employed as their colorant and printers are equipped with nozzles with smaller diameter. Accordingly, inkjet recording ink that satisfies both these requirement and other ink properties has not been developed yet.
Previous proposals for increasing the reliability of printers include improvements of the long-term preservation stability of the ink (to prevent the formulation of coarse particles due to pigment aggregations) and formulations that suppress as much as possible the increase in the viscosity of the ink when the moisture in the ink has evaporated in the vicinity of the nozzles. In addition, it is proposed to install a filter between the ink supply unit and ink discharge unit in order to prevent dust from being mixed in the ink.
Regarding the formulation of the aforesaid ink, Patent Literature 1, 2 and 3 disclose that the discharging stability is assured by regulating the viscosity when the ink is being condensed; however, with these proposed ink, it is found difficult to ensure the high image quality on regular paper.
In addition, Patent Literature 4 discloses that an ink with high viscosity (5 mPa·s to 15 mPa·s) is required to ensure high image quality. According to this proposal, it is recommended to adjust the initial vaporization speed for assuring reliability, as well as to add specific compounds as viscosity controlling agents for adjusting viscosity. However, no description is present concerning the particle size stability of the pigment, and the reliability is considered inferior in certain configuration of head and nozzle diameter when it is left to stand for a prolonged period.
Further, regarding a filter to be installed between an ink supply unit and ink discharge unit, various proposals have been made for the purpose of preventing external dust, dust within ink, precipitates from ink, and air bubbles from clogging nozzle and ink channel.
For example, Patent Literature 5 discloses a device that prevents air bubbles trapped by a filter that is installed as a means to remove dust from inhibiting the flow of the ink.
Patent Literature 6 makes a proposal to achieve both high definition and high speed. According to this proposal, smaller nozzle diameter is necessary for higher definition of image quality, which as a result requires smaller pore size of the filter for dust removal. This, combined with the increase in the supplied ink quantity due to speeding up, increases the pressure difference around the filter and decreases the maximum ink ejecting frequency. The literature states that this problem can be avoided by contriving the configuration.
Furthermore, Patent Literature 7 discloses that keeping the inertance and the flow resistance of a filter that is to be used within a certain range eliminates the clogging due to inflow of contaminants, relieves and absorbs the pressure generated when ink drops are being discharged, and therefore secures a stable discharge. However, the proposed filter consists of multiple grooves formed by etching on anisotropic crystal base, which lacks versatility. Also, the proposal misses to mention the relationship between the filter and the size and shape of colorant particles in the ink.
Patent Literature 8 points out a possibility that contaminants once trapped by a filter pass through the filter over the long-term usage of a head and repetitive absorption and discharge of ink. The proposal discloses that installing two types of filters may avoid these and hence considerably decrease poor discharge; however, this is not advisable in terms of cost.
Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 2002-337449
Patent Literature 2 JP-A No. 2000-95983
Patent Literature 3 JP-A No. 9-111166
Patent Literature 4 JP-A No. 2001-262025
Patent Literature 5 Japanese Patent (JP-B) No. 3168122
Patent Literature 6 JP-B No. 3267457
Patent Literature 7 JP-B No. 3389732
Patent Literature 8 JP-A No. 2002-273881